Conventionally, among piezoelectric devices that include piezoelectric vibrators and piezoelectric oscillators, the most common type of such devices is the surface mounting piezoelectric device bonded with a solder to a surface of an external substrate. Taking surface mounting crystal devices for instance, they may have a structure wherein a crystal element and an electronic component, for example, thermistor or semiconductor circuit element (IC), are mounted respectively on front and back surfaces of an insulated substrate portion and the crystal element is hermetically sealed with a lid. In this structure, the crystal element and the electronic component are separately housed in different spaces.
A further specific example of this structure may further have an insulated container (base). This base may have a lower recess and an upper recess. The lower recess is defined by the lower surface of a plate-shaped substrate portion (a main surface that faces the external substrate) and a frame formed on an outer peripheral part of the lower surface (lower frame). The upper recess is defined by the upper surface of the plate-shaped substrate portion (another main surface opposite to the main surface that faces the external substrate) and a frame formed on an outer peripheral part of the upper surface (upper frame). A crystal element is mounted in the upper recess of the base, while the electronic component, such as IC or thermistor, is mounted in the lower recess of the base. A plate-shaped lid is bonded to the base so as to close up the opening of the upper recess to hermetically seal the crystal element in the upper recess. The patent document 1 describes an example of the piezoelectric devices thus structured.
Referring to the cited document 1, external connection terminals are formed at four corners on the bottom surface of the lower frame in the base. A fact to be noted here is that the bottom surface of the lower frame is reduced to smaller dimensions in microminiaturized crystal devices in the size of, if they are rectangular in plan view, approximately 1.6 mm×1.2 mm or less in outside dimension. This means that an area available for the external connection terminals to be formed is accordingly smaller.
In this way it is from the next reasons that the area available for the external connection terminals to be formed becomes smaller. The electronic component to be mounted in the lower recess has at least a certain size or larger. To ensure better workability when mounting the electronic component in the lower recess, the lower recess needs to have an opening area large enough for the electronic component. This, in turn, narrows the lower frame in width and correspondingly narrows the bottom surface of the lower frame, leaving a further limited area for the external connection terminals.
As the bottom surface of the lower frame in the base is increasingly narrower, the lower frame has a proportionately lower rigidity. Then, the base may be likely to deflect under various stresses imposed thereon by the external substrate and the like.
To ensure a sufficient solder-bonding strength between such a microminiaturized crystal device and the external substrate, the external connection terminals are desirably larger but are small enough for such a limitedly narrow bottom surface of the lower frame. It follows that the external connection terminals may be essentially formed in proximity to the opening end of the lower recess as described in the patent document 2. In case the external connection terminals are too close to the opening end of the lower recess, however, the dissolved solder may flow out and reach an electronic component mounting pad (electrodes) on the inner bottom surface of the lower recess when the crystal device is mounted on the external substrate. This may increase the risks of poor insulation and other undesired events. To avoid such risks, a certain interval. i.e., an electrode-absent region where the base matrix is exposed, is desirably interposed between the inner peripheral edges of the external connection terminals and the opening edge of the lower recess.
A problem with the external connection terminal having a rectangular shape in plan view is its bent-shaped angular parts. These angular parts, where the stresses are prone to localize, may often be the origins of solder cracks. Besides that, the lower frame of the base in the microminiaturized crystal device degraded in rigidity, as described earlier, may easily cause the base to deflect under various stresses imposed thereon by the external substrate and the like. In case the base is deflected, the stresses are more likely to act on the solder-joined parts of the external connection terminals of the crystal device and to locally concentrate on the angular parts of the external connection terminals. These unfavorable events may involve the risk of generating cracks in the solder. Any crack, once it occurs in the solder, may spread, finally incurring poor connection and/or causing the crystal device to fall off.